Coating materials, e.g. polymers, are often used as protective barriers on circuit boards, electrical components, medical devices and the like. Parylene is a generic term often used to describe a class of poly-p-xylylenes which are derived from a dimer of the structure: ##STR1## where X is typically hydrogen or a halogen, e.g., fluorine, and A and B, when present, are halogens, e.g., chlorine. Due to its ability to provide thin films and conform to substrates of varied geometric shapes, parylene is ideally suited for use as a conformal coating.
Typically, parylene is applied by vapor deposition under vacuum conditions wherein the parylene monomer is condensed and polymerized directly on the surface of the object to be coated. Since the parylene monomer is not stable, the parylene dimer, as illustrated above, is used as the starting material.
Typical apparatus for carrying out parylene vapor deposition coating processes are configured to perform the coating processes in a batch mode and comprise: a vaporization zone, wherein the parylene dimer is vaporized; a pyrolysis zone, wherein the parylene dimer is pyrolyzed, i.e., heated and cleaved, to its monomeric form; a deposition chamber, wherein the objects to be coated are exposed to the parylene monomer; and a vacuum means for maintaining vacuum conditions within the deposition chamber.
Performing the coating processes in the batch mode requires that the objects be placed into the deposition chamber prior to creating a vacuum in the deposition chamber. Thus, the batch processes cannot be readily integrated with other manufacturing steps, i.e., in line processing. As a result, it is often necessary to handle the objects, both prior to and subsequent to the coating process. Care must be taken in such handling steps to avoid contamination or impurities which may lead to imperfections in the parylene coating step or subsequent processing steps. In addition, since the coating step cannot be readily conducted before the deposition chamber has reached the desired vacuum pressure, there can often be a significant process down time, i.e., period of time when coating is not be conducted. The coating time efficiency can be low, e.g., 40 to 80% of the total process cycle time.
Furthermore, in a typical batch apparatus the pressure within the deposition chamber is controlled by adjusting the heat input to the vaporization zone. Since regulating the heat input is an indirect method of pressure control, thermal lags and pressure overshoots can be encountered.
Accordingly, new continuous vapor deposition processes and apparatus are desired for coating objects with a coating material which: can be readily integrated with other manufacturing steps: would have reduced process down time in the deposition chamber and higher coating time efficiencies and would have improved process control characteristics as compared to batch processes;